(468.4659) WISE-Amsterdam - In the last few years, many PWRs experienced serious safety problems [See table I]. A higher burn-up leads to more radiation, which leads to more corrosion of the fuel rods and to deformation of the rods, such as swelling, bowing or rupture. Deformed fuel rods could hinder the flow of cooling water. Moreover, control rods stick between the fuel rods which can lead to dangerous situations. Control rods have as function to regulate the reactivity of the fuel: when the reactivity increases, the control rods are inserted deeper into the core. The control rods absorb neutrons which would otherwise lead to more fissions and to higher reactivity. Sticking control rods could eventually lead to a core melt in cases of Reactivity Insertion Accidents.
| Table I; Some PWRs with burn-up problems | |
| US | Wolf Creek North Anna South Texas 1 |
| Belgium | Tihange 3 Doel 4 |
| France | Belleville 1 Paluel 3 Nogent 1 |
| Sweden | Ringhals 1 |
| Spain | Almaraz |
The nuclear industry only set up a research and test program, after problems with high burn-up fuels and sticking control rods had appeared. In 1993, a four-year test program was started in the Cabri Research Reactor in Cadarache, France. The $52 million program is financed by research institutes in France, the US and Japan and has as objective to evaluate the maximum discharge burn-up for both enriched uranium oxide and plutonium-enriched Mixed Oxide (MOX). During the first test in November 1993 the fuel failed much sooner, and at much lower energy levels than expected, and demonstrated the need to review the safety criteria for high burn- up oxide fuels. This led the French safety organization IPSN (Institute for Safety and Protection) to refuse a license to the Electricite de France (EdF, the French electricy company) to increase the burn-up for their 900 MW PWRs from 47 to 52 MWD/kg.
The nuclear regulatory agency DSIN asked EdF to write a comprehensive report about its future fuel plans, such as fuel with higher enrichment, using MOX fuel and Recycled Uranium (REPU) fuel and for management of spent fuel.
A recent test at the Cabri reactor with MOX fuel on January 24, 1997, resulted in a violent rupture of the fuel clad. If this rupture is connected with the use of MOX, it would be bad news for utilities and the MOX fuel industry. In France the DSIN said it needs more information about the MOX fuel test, in connection with its examination of EdF's application to use MOX with a higher burn-up. The current burn-up limit on MOX fuel in France is about 40 MWD/kg, which is an economic disadvantage compared to uranium fuel. Therefore, the EdF wants to use MOX with a burn-up of 47 MWD/kg. This year another MOX fuel test is planned in the Cabri reactor. IPSN has proposed another $63 million internationally funded test program, which could begin only in 1999. Till that time, no decisions on higher burn-up will be made by safety regulators.
The EdF has ordered the replacement of fuel rods, after the problems with control rod sticking at Paluel in 1995 and 1996 and at Nogent-1 and Belleville-1 in 1996. The control rods did not drop as quickly or as far as desired or they failed to rise again. The DSIN asked EdF to conduct three control rod drop tests at Paluel and to refrain from load-following, to avoid unnecessary control rod movement. The DSIN ordered EdF to fulfill a three-page list of detailed requirements before approving the re-start of Belleville- 1. The EdF must measure the fuel deformation, the drop time, and the forces to raise and lower the control rods. The DSIN noted that there was as yet "no definitive explanation" of the problem in France or elsewhere. [NF No. 26, December 16, 1996, p.12]. The EdF says that if it cannot use fuel with burn-ups of more than 47 MWD/kg, "nuclear (energy) will have lost its bet" to be a competitive energy source at the beginning of the 21st century.
Tests on high burn-up fuel have also been done in recent years in Russia and Japan, but not yet in the US. The NRC is now planning a research program at Argonne National Laboratory in which high burn-up fuel cladding is exposed to a simulation of a Loss-Of-Coolant Accident. The NRC is concerned about the recent fuel deformation problems at Wolf Creek and North Anna. At Wolf Creek the problems increased with higher burn-up.
In the Japanese test reactor NSRR there have been another two fuel ruptures with fuel burn-up of more than 50 MWD/kg.
Internationally, the majority of a Nuclear Energy Agency (NEA) expert group concluded that more research was needed on the role of fission gas releases, on failure modes and the role of fuel-clad corrosion. A plenary NEA meeting agreed there is need for further research on the behavior of high burn-up fuel in so-called Reactivity Insertion Accidents (RIA).
Possible explanations for fuel bowing and control rod sticking are: fuel rods with more than one enrichment level, which may lead to higher-than-expected peak burn-ups and to weakening of the fuel; corrosion deposits on guide tubes. Another concern of the NEA is that increasing economic competition among utilities could lead to withholding of technical information needed to resolve problems as fuel bowing.
In the Netherlands, EPZ, the owner of the Borssele PWR, asked for a license to raise the burn-up of their fuel from 33 GWD to 50 GWd/kg. The Ministry of Economic Affairs applied the license on December 20, 1996. One of the consequences is an increase of the emission of tritium into water and air by 20 percent. In this case, too, the justification is better economics.
However, the EPZ gives no information on the amount of the profit. At the moment, Borssele is being modified to increase the safety level at a cost of 470 million Dutch guilders (about $250 million). The introduction of higher burn-up fuel, however, increases the risks again. The anti-nuclear movement should fight this dangerous development everywhere. More information about problems with high burn-up fuel is welcome.
Sources:
- Nuclear Fuel, 7 November 1994 p.1
- Nuclear Fuel, 16 December 1996 p.12-14
- Nuclear Fuel, 29 July 1996 p.9-11
- Nuclear Fuel, 10 February 1997 p.13-14
- Nuclear Fuel, 2 December 1996 p.16-18
- Nuclear Fuel, 16 December 1996 p.12-14
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